1. Field of the Invention
The present invention relates to the field of analog wave form digital encoding, and more particularly relates to an improved method and apparatus for generating delta modulation data signals.
2. Background Art
Delta modulation is a well known method for encoding analog wave forms into digital form, which may be implemented in a number of ways. Characteristics of delta modulation make it preferable over pulse code modulation in various kinds of applications. For example, certain types of delta modulation systems are particularly suited for operating situations characterized by high error rate.
One such high error rate application involves the encoding and recording of audio frequency information on an optical disc, for example a video optical disc. By encoding one or more messages of audio frequency information in digital form and recording the encoded audio information on the disc, the audio information can be read off the disc and stored for controllable access and playback during, for example, the stop-motion or "freeze frame" playback of a single frame of video from the disc. It is known, however, that at the present state of technology commercial quality video discs have significant levels of short duration signal drop-out. Delta modulation is, therefore, a preferred technique for encoding audio information in such applications. Specifically, adaptive delta modulation ("ADM") provides performance characteristics particularly suited for this purpose.
In connection with the encoding and storage on a recording medium of audio information for later playback, the storage capacity of the medium becomes a significant consideration. For example, while the standard video optical disc is a remarkably high density storage medium for its size, its storage capability is limited and, as a consequence, stop-motion audio encoding formats being developed for the industry utilize delta modulation at a sample rate of between 12 and 28 kilohertz. This range of sample rates reflects a compromise between desired audio message duration and acceptable fidelity. The audio fidelity of decoded adaptive delta modulation audio data, even at 28 kilohertz sampling rate, is noticably degraded from, for example, standard commercial television audio.
It is therefore desired to improve the level of fidelity provided by a delta-modulation system to provide, for example, improved audio playback from a recording medium of ADM encoded audio without increasing the sample rate of the recorded audio information.
Improvements have been suggested. However, the improvements frequently reflect complex theoretical considerations which require expensive implementations for both encoding and decoding. In audio information encoding and recording applications, for example in connection with video optical discs, it is desired to keep decoder implementations as simple as possible in order to keep the costs of optical disc players within the range of consumers. However, since recording media such as optical discs are usually mass produced, the impact of a relatively high cost for encoding audio information for a particular recording may be minimal. It is therefore desired to have an improved delta modulation system which provides optimized delta modulation encoding for relatively inexpensive decoding.
The present invention provides such a system and allows optimization of the encoding of a waveform into ADM data while providing flexibility in the approach to said optimization.